AUGUSTA, Maine — Maine’s schools chief, who brought such sweeping education policy changes to the state as the A through F grading system for schools themselves, resigned Friday to work for a national nonprofit organization.

Commissioner Stephen Bowen will join the Washington-based Council of Chief State School Officers where he said he will assist other states in putting in place public education system policy changes.

Governor Paul LePage praised Bowen’s leadership during his 2½ years as commissioner, saying that education has ‘‘undergone its most significant transformation in the state’s history’’ under his direction.

‘‘It is encouraging that a national organization committed to educational excellence has recognized Commissioner’s Bowen passion and dedication for improving schools and student outcomes,’’ the Republican governor said in a statement.

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But Bowen’s policies, like charter schools, have also consistently faced sharp criticism from Democrats and teachers unions.

Bowen, who was tapped to become commissioner in 2011 after years as an education policy analyst at the conservative think tank the Maine Heritage Policy Center, said the council approached him to help ‘‘take Maine’s transformation nationwide.’’

‘‘The offer was not one I took lightly, but ultimately the opportunity to take what is working here in Maine to scale was one I could not refuse,’’ he said in a statement.

Bowen’s last day with the state will be Sept. 12 and an acting commissioner will be chosen soon, LePage’s office said.

Bowen brought a number of contentious education policies to the state, such as legalizing charter schools and making Maine the 14th state to give letter grades to schools. LePage’s office said ‘‘dramatic reforms’’ have made Maine a leader.

But the Maine Education Association, which represents more than 25,000 Maine educators and school officials, has attacked the school grading system, saying it awards wealthy districts and penalizes schools with fewer resources.

Senator Rebecca Millett, Democrat of Somerville and Senate chairwoman of the Education Committee, said Bowen was an ‘‘intelligent and committed public servant’’ but called many of his policies and proposals ‘‘misguided and unproductive.’’

‘‘It is my hope that we can use this opportunity to find an education commissioner who is committed to strengthening Maine’s public schools,’’ she said in a statement.

But Bowen said he is proud of what the department has accomplished and urged the state to continue this ‘‘transformational work on behalf of Maine kids.’’

‘‘Maine is a state the nation is now looking to as a leader in meaningful education reform and I am confident that the reforms we have advanced, both in terms of education policy and in terms of how the department does its work to support schools and students, will lead to improved student outcomes,’’ he said.

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